Some data storage systems, such as disc drive systems, use a motor to change the position of storage regions with respect to a mechanism to read and/or write to the storage regions.
For example, a disc drive data storage system typically includes one or more discs that store information using a magnetic medium. A spindle motor rotates the discs, and one or more read/write heads read information from and write information to memory regions on the discs.
In order to accurately position a head with respect to a storage region on a disc, variations in the rotational speed of the disc need to stay within acceptable limits. FIG. 1 shows a spindle motor system 100 with a stator 110 having stator windings 120-1 to 120-6 around six radially distributed stator poles 130-1 to 130-6. A rotor 140 includes a number of permanent magnets 150 each with poles 150-N and 150-S. Current flow through different ones of the stator windings 120-1 to 120-6 generates electromagnetic fields, which interact with the fields generated by the permanent magnets 150. Torque generated by this electromagnetic interaction causes rotor 140 to rotate with respect to stator 110.
Once spindle motor 100 has been accelerated to operational speed, a spindle motor control system is used to maintain the operational speed. During each rotation of the spindle motor, some energy is dissipated (e.g., as heat due to friction). Therefore, windings 120-1 through 120-6 need to be energized to provide the required torque to maintain the desired rotational speed.
As noted above, energy is provided to motor 100 by applying voltages of appropriate duration to the windings 120-1 to 120-6 at appropriate times. For example, at a particular time t0, current may be generated in winding 120-1 by applying a positive voltage (e.g., +5 volts) to winding 120-1. Also at t0, current may be generated in winding 120-3 to flow in the opposite direction in winding 120-3 by applying a ground or negative voltage to winding 120-3. Winding 120-2 may not be energized at time t0. However, as the permanent magnets 150 of rotor 140 rotate with respect to winding 120-2, the changing magnetic flux generates a back electromotive force (BEMF), which induces current flow in winding 120-2. The induced current may be detected and used in a feedback system to maintain the desired rotational speed.
In existing disc drive systems, the BEMF generated in a single winding is typically used to provide feedback for the spindle motor speed control system. A measured revolution time τMeas is determined by monitoring the BEMF signal in the winding for a complete revolution of the rotor. τMeas is compared to a reference time τRef, where τRef is the revolution time corresponding to the desired speed. The feedback system generates current in the windings to produce the necessary torque based on the difference between τMeas and τRef.